The current documentation link below does not have any indication / mention of "Execute" execute permissions needed to the open report.
External Reports that are stored on the Agent cannot be opened by users having only authorization flags R S P (Read, Executions, Open Report). In this case it requires File Transfer to deliver the files marked as external reports from the agent to the user And File Transfer requires Execution (X Flag) Authorization for the user profile.
21.0.8, 21.0.9
Known defect.
Missing documentation is scheduled to be added in version 21.0.10 tentatively scheduled for end of April. (Version and timeline may change depending internal testing)
We will add a note in the 21.0.10 Documentation, under Downloadding External Reports as below:
note - as the download is performed through a File Transfer object - The user needs execute permissions additionally to the open report.
Reference Defect AE-34818